1. Field of the Invention
The present invention relates to an Fe-based alloy powder adapted for sintering and having superb compressibility and corrosion resistance, and an Fe-based sintered alloy having superb wear resistance. The Fe-based alloy powder and sintered alloy are useful to make sintered component parts, such as valve seats and piston rings for internal combustion engines, collars for exhaust systems, and the like. The present invention also relates to a process for producing the Fe-based sintered alloy.
2. Description of Related Art
Japanese Unexamined Patent Publication (KOKAI) No. 56-154,110 discloses a conventional alloy for making the valve seats. The conventional alloy is prepared by adding an intermetallic compound, such as ferromolybdenum (e.g., Fe--Mo) or ferrochromium (e.g., Fe--Cr), or an Fe--C--Cr--Mo--V alloy, to an Fe--C--Co--Ni--based alloy or an Fe--C--based alloy in order to improve the wear resistance.
Japanese Unexamined Patent Publication (KOKAI) No. 60-224,762 discloses a sintered alloy. In this sintered alloy, Fe-based hard particles containing Cr, Mo and V are dispersed in the Fe--C matrix containing Cr and Mo in order to improve the wear resistance and the harshness against mating parts.
Japanese Unexamined Patent Publication (KOKAI) No. 62-202,058 discloses another sintered alloy. In this sintered alloy, hard particles including FeMo and FeW are dispersed in the Fe--C--Co--Ni matrix, and a Pb alloy or the like is impregnated thereinto in order to enhance the wear resistance.
The alloys for making the valve seats are required to have the corrosion resistance and the heat resistance in addition to the wear resistance. In the aforementioned sintered alloys, the hard particles mainly effect the wear resistance, and the matrices mainly effect the corrosion resistance and the heat resistance. Thus, the hard particles and the matrices cooperatively effect the durability securely.
Recently, in the field of automobile engines, the following improvement requirements have been demanded more strongly than ever: the extension of longevity, the increment of output, the increment of speed, the countermeasure against exhaust gas, the countermeasure against fuel consumption, and the like. Therefore, the engine valves, the valves seats, and the like of the automobile engines must inevitably withstand much severer service environments than ever. Accordingly, they are required to have further improved heat resistance and wear resistance, and they are also required to have enhanced corrosion resistance at elevated temperatures.
When forming the matrices of the conventional Fe-based alloys for making the valve seats, each ingredient powder of the alloying elements, such as Ni, Co, Mo, and the like, is mixed with an iron powder to make a mixed powder, i.e., a raw material. Thereafter, the resulting mixed powder is formed and sintered, thereby diffusing Ni, Co, Mo, and the like into the iron. For instance, as set forth in Japanese Unexamined Patent Publication (KOKAI) No. 3-158,444, an Fe--Cr powder, a carbonyl powder, a Co powder, an Mo powder and a graphite powder are prepared as raw material powders for making an Fe-based sintered alloy for valve seats. The raw material powders are then mixed with hard particles to produce valve seats made of an Fe-based sintered alloy in which the hard particles are dispersed in the Fe-based alloy matrix.
However, it is hard to completely diffuse the alloying elements into the iron. As a result, it is difficult to improve the physical properties of the resulting Fe-based sintered alloys in proportion to their addition amounts.
Hence, one might think of alloying iron and the alloying elements in advance in order to effectively produce the advantageous effects resulting from the addition of the alloying elements. However, when alloying iron and the alloying elements in advance, the resulting Fe-based alloy powders exhibit deteriorated compressibility because of the solution hardening, thereby making it difficult to highly densify the green compacts. As a result, it is disadvantageous when improving the products made of the Fe-based alloy powders in terms of the durability.